The present invention relates to the design of a permanently bistable twisted nematic liquid crystal display, and in particular to such a display has two stable twist states with no applied voltage. The display can be switched between these two stable states by the application of special voltage pulses. Due to this bistability, no power is needed to maintain the display, once it is formed.
There are many types of bistable liquid crystal displays (LCD). Among them, the bistable twisted nematic (BTN) LCD has one of the best optical properties with good contrast and brightness. This display relies on adjusting judiciously the cell thickness to liquid crystal pitch (d/p) ratio to produce the metastable twist states.
Bistable twisted nematic (BTN) liquid crystal displays (LCD) have been around since the early discovery by Berreman et al (D. W. Berreman and W. R. Heffner, Appl. Phys. Lett.37, 109 (1980)). In the early prior art, the bistable twist states are 0 and 360xc2x0 twist respectively. Recently, Kwok et al (full reference) extended these BTN to the general case of xcfx86 and xcfx86+2xcfx80, where xcfx86 can be optimized to yield better optical properties such as contrast and on-state brightness.
In this latter BTN, xcfx86 can be optimized to produce excellent optical properties. However, the major problem of this BTN is that the xcfx86 and xcfx86+2xcfx80 twist states are metastable with short lifetimes. Indeed, the intermediate xcfx86+xcfx80 twist state is more stable and both the xcfx86 and xcfx86+2xcfx80 states will decay to it in a matter of seconds.
There are several variations of the basic BTN. Hoke et al (C. D. Hoke and P. J. Bos, Soc. Info. Disp. Symp. Digest, 29, 854 (1998)) use a polymer wall trying to stabilize this BTN. The results are not satisfactory as the two twist states are still short-lived. Bryan-Brown et al (GB9521106.6) demonstrated a grating aligned bistable nematic display that can be switched by sub-millisecond pulses and has infinite time memory. The display has been produced and disclosed. Its practicality is in doubt because of the need of a special surface structure in the construction of the display. Switching from one state to the other is also not easy.
Dozov et al (I. Dozov, M. Nobili and G. Durand, Appl. Phys. Lett. 70, 1179 (1997)) investigated a surface-controlled bistable nematic display using simple planar monostable anchoring. A high pretilt angle and a small cell gap are necessary to produce the bistability and switching between the 0 and xcfx80 twist states. This display is permanently stable in that the 0 and xcfx80 states do not decay to any intermediate states. It is similar to the xcfx86 and xcfx86+2xcfx80 BTN.
According to the present invention there are disclosed designs of permanently bistable twisted nematic liquid crystal displays with optimized optical properties. The stable twist states differ by an angle of xcfx80, or 180xc2x0. They are denoted xcfx86 and xcfx86+xcfx80. The aligment of the xcfx86 and xcfx86+xcfx80 twists are given by the rubbing condition that favors the xcfx86 twist. The d/p ratio is adjusted to favor the xcfx86+xcfx80/2 twist. This enables both the xcfx86 and xcfx86+xcfx80 twist states to be stable. The d/p ratio favored xcfx86+xcfx80/2 twist state is not stable under the given rubbing conditions. Hence the lifetimes of the xcfx86 and xcfx86+xcfx80 states are infinite.
The switching between the xcfx86 and xcfx86+xcfx80 twist states are effected through many techniques, such as having a small cell gap, or having a three electrode structure. One possible example of switching uses a triode structure. However, it is not essential to the present invention and other schemes are possible.
The new BTN can either be transmittive requiring two polarizers, or reflective requiring just one polarizer. In general, the optical properties depend on xcex1 and xcex3, which are the input and output polarizer angles, with respect to the input director of the liquid crystal cell, and the twist angles xcfx861 and xcfx862 of the two bistable twist states. The optical properties also depend on d, the liquid crystal cell thickness and xcex94n is the birefringence of the liquid crystal material used. With all these values defined, the optical properties and construction of the liquid crystal display is uniquely defined. A perfect optimization procedure is given to give the exact values of the construction of the BTN to give the best optical properties.
It is also possible to construct the BTN in the reflective mode with just one top polarizer. In this case, there is no output polarizer and the display is easier to construct and more economical to make. We also will give the construction parameters for such reflective BTN to obtain the best optical properties. An important aspect of the invention is that the bistable twist states differ by an angle of 180xc2x0. rather than 360xc2x0 as in the previous cases.